The present invention relates to an improved medium voltage (MV) switching device. More particularly, the present invention relates to an improved MV switching device, such as a circuit breaker, a disconnector, a recloser or the like, which is particularly useful for the use in medium-voltage power distribution networks, i.e. approximately for voltage values between 1 KV and 50 KV.
As it is known, a MV switching device normally comprises a control unit and an actuation chain for actuating, i.e. coupling/uncoupling, the electric contacts of the MV switching device. The control unit receives opening/closing commands, for example from a protection device or an operator. Based on these commands, it sends control signals to the actuation chain that provides for operating the electric contacts of the switchgear, so as to bring it in the desired operating state, i.e. an opening state or a closing state.
Initially, MV switching devices of the state of the art were generally of the mechanical type, i.e. provided with an actuation chain which basically adopts suitable mechanical arrangements including for example springs or leverages.
Recently, electromagnetic switching devices, i.e. devices provided with an electromagnetic actuation chain have been introduced. An electromagnetic actuation chain generally comprises an electromagnetic actuator operatively connected to the electric contacts of the switchgear by means of a kinematic chain. The electromagnetic actuation chain is generally supplied by power supply means, which normally draw electric power from a separated power distribution line with voltage values lower than 1 KV, i.e. a power distribution line of the low voltage (LV) kind. Power supply means comprise generally a plurality of capacitor banks, connected to a power charger, which provides electric power to the capacitor banks from a LV voltage source, in order to maintain a suitable level of stored electric energy.
Once the control unit receives an opening/closing signal, it commands a power circuit to provide electric power to the electromagnetic actuator. Meanwhile, the power charger provides electric power to the capacitor banks in order to restore a suitable level of stored electric energy.
MV switching devices of the electromagnetic type have represented a remarkable improvement in the state of the art, particularly in terms of costs and performances. Nevertheless, they still have some drawbacks.
These drawbacks are represented by the fact that some important requirements, which are commonly felt as important by the user, are not fully satisfied. A first important requirement consists of the possibility of supplying electric power to the electromagnetic actuation chain, in a substantially continuous manner.
For example, the MV switching device should be able to maintain, if needed, the MV switching device in a certain operating state, particularly in a closing state, for an indefinite period of time and should be able to change its operating state at any time. A second important requirement consists of the possibility of having a substantially self-supplied MV switching device. In practice, the operation efficiency of a MV switching device should not depend on an external power supply.
Unfortunately, in case of fault of the power supply means, an electromagnetic MV switching device is able to perform opening/closing operations, only for a short period of time, due to the fact that the power supply means basically depend on external power source, i.e. a LV power distribution line. Thus, only a single OCO (opening/closing/opening) switching cycle can be performed normally. In fact, in case of power supply failure, the MV switching device is able to operate until there is a certain amount of energy stored in the capacitor banks. After capacitor banks are discharged, no further operation is possible till the power supply is not restored. Thus, for example, it is difficult to perform an O-0.3 s-CO-15 s-CO (opening/0.3 s delay-closing/opening-15 s delay-closing/opening) switching cycle, which is commonly required by the user. This can be achieved, for example, over-sizing the capacitor banks but this is quite expensive.
This fact leads a reduced safety and reliability of the electromagnetic MV switching devices, especially if compared with MV switching devices, which adopt a purely mechanical actuation chain. Thus, the practice has shown that, often, the electromagnetic MV switching device has to be associated to continuity power supply systems that are relatively expensive and, therefore, make the electromagnetic MV switching devices less competitive and limited to high end applications.
Therefore, the main aim of the present invention is to provide an electromagnetic medium voltage switching device, i.e. 0adopting an electromagnetic actuation chain, which allows overcoming the drawbacks described above.